The invention relates to a retainer bearing for retaining a rotor shaft of an electric machine, wherein the retainer bearing has a bearing outer ring and a bearing inner ring. Furthermore, the invention relates to an electric machine, in particular a turbomachine, comprising at least one such retainer bearing. The electric machine has, in particular, magnetic bearings for bearing the rotor shaft for operation purposes.
Electric machines, in particular rotating electric motors and generators, require bearings for bearing the rotor shaft. Rolling bearings, such as ball bearings or roller bearings, are usually used for this purpose.
Such bearings are no longer advantageous for large electric machines, that is to say, for example, machines with a mass of more than one ton and a rated electrical power of more than 500 kW, in particular of a few megawatts. This applies, in particular, to electric machines which have a maximum rotation speed of more than 4000 rev/min, as is the case, for example, in compressors or pumps. The reason for this is wear of the rolling bearings which increases more than proportionately as the rotation speed of the electric machine increases. The result is regular replacement of the rolling bearings and stoppage of the electric machine and the system components connected to it.
A further disadvantage of the rolling bearings is that the rolling bearings have to be regularly oiled or lubricated. In nature protection or water protection areas, this means that electric machines with rolling bearings can be operated for the purpose of driving compressors, pumps or the like only under strict environmental conditions.
Active magnetic bearings are used instead of rolling bearings in order to solve the abovementioned problems. Such bearings avoid the disadvantages of rolling bearings, such as friction, poor damping and wear. On account of the continual advances in the field of computer and control engineering and power electronics, such active magnetic bearings are highly dynamic and very stable in terms of control. Contact-free, wear-free and stable bearing is possible at very high rotation speeds. A further advantage is that the rigidity and damping of the active magnetic bearing can be electronically adjusted. On account of lubrication which is otherwise required being dispensed with, active magnetic bearings are free of oil and grease and can therefore be used particularly in environmentally sensitive areas of use. An air gap in the range of from 0.1 mm to 0.5 mm is maintained between the magnetic bearing and the rotor shaft to be borne during operational use.
Since an active magnetic bearing may break down due to the system, retainer bearings can be provided, which retainer bearings can hold the rotor shaft when there is no magnetic bearing or generally when the electrical system is switched off. To this end, the retainer bearing has a slightly larger inside diameter compared to the diameter of the rotor shaft, and so the rotor shaft does not touch the retainer bearing in the case of magnetic bearing during operation. The retainer bearing is usually accommodated in the region of the respective end of the rotor shaft in the stator housing of the electric machine. If, in contrast, the rotor shaft is in the form of a hollow shaft, the outside diameter of the retainer bearing is slightly smaller than the inside diameter of the hollow shaft, and so the rotor shaft can retain the rotor shaft from inside.
When a magnetic bearing breaks down, the rotor shaft falls into the retainer bearing. This creates the risk of the rotor performing a so-called “backward whirl” and rolling away along the inner surface of the retainer bearing. In contrast to the rotationally synchronous rotor movement, in the case of which the deflection of the rotor is synchronous with the revolving unbalance excitation, the rotor passes through the orbit in the opposite direction to the rotation of the rotor with a very large amplitude in the case of the backward whirl. In this case, a rotationally synchronous proportion with a very much smaller amplitude superimposes itself, and so an elliptical orbit is produced. This results in very strong forces acting on the rotor and stator, which forces can destroy the machine in an extremely short time.
The risk of the backward whirl increases as the coefficient of sliding friction between the rotor shaft and the retainer bearing increases. The risk can be avoided by virtue of the use of rolling bearings on account of the very small coefficient of rolling friction.
However, rolling bearings are not suitable for retaining large and heavy rotors in the event of a breakdown since there is only one point of contact between the rolling bodies and bearing rings in the case of ball bearings and only one line contact, together with the resultant very high surface pressures, in the case of roller bearings. These high loads can damage the rolling bearings and block the bearing.
For this reason, dry friction bearings are currently used as retainer bearings for large and heavy rotors. In the case of dry friction bearings, there is frictional contact directly between the rotating rotor shaft and the friction layer of the dry friction bearing. It is possible to largely avoid a backward whirl by suitable material selection, for example by using special bronze alloys for the friction layer.
One disadvantage of this solution is that it leads to high surface pressures in the retainer bearing when the rotor shaft first strikes. The reason for this is the different diameter of the dry friction bearing and the rotor shaft. In addition, the simultaneously acting full friction speed occurs in the contact region. This extreme load leads to the friction surface, which primarily comprises a bronze alloy, melting and being deposited on the rotor shaft when the rotor shaft stops in the retainer bearing. This makes it necessary to mechanically rework the rotor shaft after the rotor shaft has stopped in the retainer bearing a few times. Furthermore, it is necessary to switch off the electric machine and the system components connected to it.